Even assuming Gary's body is somehow immune to the large list of physics-related problems that immediately spring to mind for super-concentrated masses, it doesn't look good for anyone involved.
Think of the Earth + Gary as a single system. That system has the same total mass as it always had, only the center of mass has moved. At astronomical scales, that's not far enough to seriously perturb the orbit of the planet or the moon. You might shave a few fractions of a second off of the next leap year, but nothing catastrophic here. Just don't keep it like this for a long time.
The problems are on the smaller scales. Particularly, imagine if you were on the opposite side of the planet from Gary. The mass of the Earth+Gary system is the same, but your distance from the center of mass is now roughly twice as far as it was a moment ago. That's a sudden and instantaneous drop in the gravitational force applied. You were standing on the ground, which means the force of gravity pulling you down compressed the ground, and this was balanced by the recoil forces of the ground pushing back up on you. Assuming the Earth remains a solid, there will be an instant where the recoil force will remain the same, but the gravitational force will be reduced. The net effect would be like placing an action figure on a table and then bumping the underside of the table. The poor guy would pop up in the air and fall back down. Whether this equates to an awkward stumble or a three-story toss is left to the storyteller. At a minimum, you'll have lots of fragile knick-knacks broken.
If the Earth does not remain a solid (if it becomes "Earth-colored air" like your gold block example), then the poor folks on the other side of the planet are in a really rough spot. They'll be accelerated towards the center of gravity of the Earth+Gary system, and with no solid ground to hold them up, they'll move into the interior of the planet. When Gary puts the mass back, they'll be stuck underground, inside the planet. Not only would this be an irritating inconvenience, you'd have all sorts of problems with two things trying to occupy the same space at the same time.
Gary is in a particularly rough spot, though. Gravity normally pulls him towards the ground, and that force results in friction between his feet and the ground. That friction is what allows him to stand in place when a force pushes on him. Once the mass gets redistributed, the Earth has zero mass. There's no longer any force to generate friction between him and the Earth. Without anything holding him in place, the impact of Superman's punch would send him sailing off into the distance. He'd lose contact with the Earth, his powers would no longer work, the mass would return to normal, and he'd fall to the ground with a lot of horizontal momentum. When he hit the ground his powers would re-activate and the Earth would go massless again. Gary's velocity vector at that point has a downward component, so his momentum carries him into the interior of the Earth. There's nothing here with mass to impede his motion, so he continues on that vector until he crosses back out of the planet somewhere else, loses contact with the Earth, gravity returns, he falls back towards the Earth, his powers re-activate, etc, etc. It might actually take Gary a rather long time before he comes to a halt. He's now like a stone skipping across a pond, if that stone could skip on the top and bottom surfaces of the water.
Luckily, there's an easier solution to all of this. If you're going to get in a fight with Superman, let him punch you. Block the punch and the instant he makes contact with you, take away all of Superman's mass. A punch thrown by a massless assailant will have zero kinetic energy behind it. Kinetic energy must be conserved so it will be absorbed into Gary along with the mass but when it's evenly distributed throughout his body, it's nothing he can't handle (that's one way bullet-proof vests work).